The focus of this revised competitive renewal is to understand the neurobiological substrates that link single gene mutations to cognitive deficits. Currently, there is no effective treatment for the cognitive disruptions that define neurodevelopmental disorders (NDDs). In order to develop therapeutic strategies that improve the lives of affected individuals, it is imperative to understand how the neurobiological substrates that underlie reduced cognition and behavioral adaptations are damaged in these disorders. Our work during the last budget period established Syngap1 Heterozygous KO mice as a robust model that will advance our understanding of how cognitive and behavioral disruptions arise in NDDs. In particular, we established that pathogenic Syngap1 mutations damage the developing brain by altering the maturation rate of forebrain excitatory neurons. We also succeeded in connecting alterations to forebrain pyramidal neuron maturation in Syngap1 mutant mice to reduced cognitive ability. However, it remains unclear how altered neuronal maturation actually degrades cognitive ability. Our studies in this revised proposal are designed to explore the hypothesis that there is a spatial pattern of circuit assembly errors in Syngap1 mutants. We believe that alternated maturation of forebrain pyramidal neurons disrupts the assembly of cortical circuits that underlies cognitive ability, which is hypothesized to be a key neurobiological substrate that connects pathogenic Syngap1 mutations to altered cognition. The impact of our expected results is that the pattern of circuit assembly errors could define the particular cognitive endophenotype displayed by Syngap1 mutant mice, which would provide significant insight into the etiology of reduced cognition due to a single gene disruption. In addition, the patterns of assembly errors in Syngap1 mutants could be used as a comparative benchmark to understand similarities and/or differences in patterns of altered circuitry in other monogenic forms of NDDs. The long-term goal of this proposal is to identify the cells in the brain that are most severely impacted by pathogenic Syngap1 mutations. Once we understand the cellular origins of the disorder, we will have a much better entry point for gaining insight into te molecular perturbations that trigger the systems level dysfunction that directly leads to reduced cognitive ability. At the conclusion of the next requested budget period, we believe we will have reduced this disorder down to a relatively selective pool of neurons that are particularly sensitiv to pathogenic Syngap1 mutations. When this information is combined with the critical period information discovered in the previous budget period, we will have the ideal entry point for assessing how pathogenic Synagp1 mutations disrupt the molecular pathways that control growth and maturation of developing forebrain pyramidal neurons. In the subsequent budget period, we would then begin studying how altered developmental Syngap1 expression in this pool of neurons triggers changes in neuronal maturation.